The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the atmosphere through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification (DeNOx) catalyst is used in one type of such a catalytic converter, and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbons (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced by the DeNOx catalyst through an oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst has been used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when an air/fuel ratio is lean, and releases the absorbed NOx when the air/fuel ratio is rich, and reduces the released NOx and the NOx contained in the exhaust gas when the air/fuel ratio is rich (hereinafter called ‘regeneration of the LNT’).
Since general diesel engines are operated at a lean air/fuel ratio, however, it is required to artificially adjust the air/fuel ratio to be rich in order to release the absorbed NOx from the LNT. For this purpose, a time for releasing the NOx absorbed in the LNT (i.e., regeneration timing) should be accurately determined.
In addition, if a temperature of the exhaust gas is high (e.g., the temperature of the exhaust gas is higher than 400° C.), the LNT cannot remove the NOx contained in the exhaust gas. In order to solve such problems, a selective catalytic reduction (SCR) catalyst is used together with the LNT.
Here, when the air/fuel ratio of the engine is rich, the content of a non-combusted fuel that is included in the exhaust gas and passing through the LNT is increased. Accordingly, we have found that an oxygen concentration must be controlled to activate an oxidation-reduction reaction of a selective catalytic reduction (SCR) catalyst.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.